adding custom cosmetics...

[stone roost]

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[unkempt topaz]

The answer is, using mods. Or Commands.

In vanilla the only way without replacing things, is using custom model data (run !faq custom-model-data for more info on that)
Or using a mod like Optifine, to allow you to change models based on an itemb eing renamed.
Or making a mod that just allows you to add new stuff without limits.

Custom model data would likely be the way to go.